(1) Field of The Invention
This invention relates to a composition made from vinylidene chloride polymers or copolymers. More particularly, this invention relates to a vinylidene chloride composition having enhanced thermal stability and improved oxygen barrier properties, and to mono-layer and multi-layer packaging films having a layer of vinylidene chloride polymer or copolymer and to a method for making the films.
(2) Description of the Prior Art
Flexible thermoplastic packaging films made of vinylidene chloride copolymer, hereinafter referred to generally as PVDC (such materials are also commonly known as "saran") have been used for a long time to package food products which include cheese, fresh and processed meats, and a wide variety of other food and nonfood items. An example of such films is disclosed in U.S. Pat. No. 2,919,059 to Sporka.
A later and more satisfactory method of producing a multi-layer film having a PVDC layer is disclosed in U.S. Pat. No. 4,112,181, to Baird. This patent describes a method of co-extruding a tubular film wherein the walls of the tube have at least three layers, including a PVDC center layer. The tubular film is subsequently biaxially oriented.
Another satisfactory method of producing a multi-layer film having a PVDC barrier layer is disclosed in U.S. Pat. No. 3,741,253 to Brax et al, which discloses a multi-layer, biaxially oriented film. This film is made by an extrusion coating process in which a substrate layer or layers of a polymer such as polyethylene or ethylene vinyl acetate copolymer is extruded in the form of a tube, cross-linked by irradiation, and inflated. A layer of PVDC is extrusion coated onto the inflated tubing, and another layer or layers of polymer is simultaneously or sequentially extrusion coated onto the PVDC. After cooling, this multi-layer tubular structure is flattened and rolled up. Then, the tube is inflated, and heated to its orientation temperature, thereby biaxially orienting the film. The bubble is rapidly cooled to set the orientation. This process produces a shrinkable barrier film with low oxygen permeability.
It is standard practice that for PVDC to be successfully extruded at commercial rates, but to do so the vinylidene chloride copolymers must be stabilized and plasticized. One plasticizer/stabilizer combination is the liquid stabilizer, epichlorohydrin/ bisphenol A, an epoxy resin, and the plasticizer, 2-ethyl hexyl diphenyl phosphate. Other known liquid stabilizers include epoxy compounds, such as epoxidized linseed oil and epoxidized soybean oil and citrates. Note also that U.S. Pat. No. 4,714,638 to Lustig et al. discloses using dibutyl sebacte or epoxidized soybean oil as a conventional plasticizer for PVDC-MA barrier films. Also of interest is U.S. Pat. No. 4,320,175 to Hisazumi et al, which shows a PVDC layer of reduced viscosity heat-pressure laminated to another PVDC layer. Epoxy compounds are used as stabilizers for the PVDC. For clarity, it is noted that sometimes the art refers to the epoxy compounds as plasticizers instead of stabilizers.
In Canadian Patent No. 968,689, to Gillio-tos et al, the effect of plasticizers, such as dibutyl sebacate on the barrier properties of a PVDC barrier layer in a multi-layer thermoplastic packaging film is described. First, the Gillio-tos et al patent discloses that homopolymers of vinylidene chloride cannot be converted into film by conventional extrusion techniques because they decompose very rapidly at the temperature of extrusion. Second, by copolymerizing vinylidene chloride with minor amounts of one or more other monomers such as vinyl chloride, methyl acrylate, etc. it is possible to produce copolymers which, when mixed with suitable plasticizers, can be extruded into films which can be oriented by stretching to give heat shrinkable films for packaging, particularly for packaging food. As stated in Gillio-tos et al, vinylidene chloride copolymers need to be plasticized so that they can be extruded and stretched into oriented films at commercial rates. The greater the proportion of plasticizer, the lower the viscosity and the easier the polymer is to extrude and orient and the better the abuse resistance of the final product. On the other hand, the oxygen permeability of the final product also increases with increasing plasticizer content and for many purposes, especially packaging food, it is vital that the oxygen permeability is low. For current commercial practices permeability below 100 cc./24 hours/m.sup.2 /atmosphere at room temperature is expected and a permeability below 50 is highly desirable. The test for oxygen transmission is conducted as per ASTM D3985.
Also of interest are the following patents which show various additives for PVDC. U.S. Pat. No. 4,401,788 to Hiyoshi et al, shows a PVDC latex with an anionic surfactant, a nonionic surfactant, and a cationic surfactant. U.S. Pat. No. 4,418,168, to Johnson, shows stabilizing particulate PVDC by coating thereon a dispersion of tetrasodium pyrophophate (TSPP) in liquid plasticizer such as epoxidized soybean oil and epoxidized linseed oil. German Patent No. 3,605,405, shows five-layer films with a PVDC core layer, wherein the EVA adhesion layers on each side of the PVDC core layer contain TSPP to heat stabilize the PVDC core layer. U.S. Pat. No. 3,524,795, to Peterson, shows multiple layer PVDC films and mentions typical plasticizers for PVDC being acetal tributyl citrate, epoxidized soybean oil, and dibutyl sebacate.
U.S. Pat. Nos. 4,686,148, and 4,698,111, both to Havens disclose the addition of glycerin and a plasticizer such as epoxidized soybean oil to a vinylidene chloride polymeric composition to lower oxygen permeability and enhance thermal stability of a film having a plasticized layer of vinylidene chloride polymer.
It is an object of the present invention to provide a vinylidene chloride copolymer composition which can be extruded with reduced plasticizer and/or stabilizer.
Another object of the present invention to provide a vinylidene chloride copolymer composition which has low oxygen transmission.
It is another object of the present invention to provide a film wherein the PVDC layer is thick enough to be an effective gas barrier but thin enough so that it is not adversely affected to any significant extent by irradiation used to cross-link layers which are cross-linkable and adjacent to the PVDC layer in a multi-layer film.
Still another object of the present invention is to improve the thermal stability of vinylidene chloride copolymers and to lessen their tendency to degrade while being extruded.
Yet still another object of the present invention is to increase the extrusion rate of vinylidene chloride copolymer compositions.
A still further object of the present invention is to reduce the stickiness of vinylidene chloride copolymer compositions with respect to metal surfaces in contact with the material during its processing.